Accelerometers are commonly employed to measure the second derivative of displacement with respect to time. In particular, linear accelerometers measure linear acceleration generally along a sensing axis. Linear accelerometers are frequently employed to generate an output signal (e.g., voltage) proportional to linear acceleration for use in any of a number of vehicle control and motion based systems. For example, the sensed output from a linear accelerometer may be used to control safety-related devices onboard an automotive vehicle, such as front and side impact air bags and rollover detection devices. According to other examples, accelerometers may be used in automotive vehicles for vehicle dynamics control and suspension control applications.
Conventional linear accelerometers often employ an inertial mass suspended from a support frame by multiple support beams. The mass, support beams and frame generally act as a spring mass system, such that the displacement of the mass is proportional to the linear acceleration applied to the frame. The displacement of the mass generates a voltage proportional to linear acceleration which, in turn, is used as a measure of the linear acceleration.
One type of an accelerometer is a micro-electromechanical structure (MEMS) sensor that employs a capacitive coupling between interdigitated fixed and movable capacitive plates that are movable relative to each other in response to linear acceleration. An example of a capacitive type single-axis linear accelerometer is disclosed in U.S. Pat. No. 6,761,070, entitled “MICROFABRICATED LINEAR ACCELEROMETER,” the entire disclosure of which is hereby incorporated herein by reference. An example of a capacitive type dual-axis accelerometer is disclosed in U.S. application Ser. No. 10/832,666, filed on Apr. 27, 2004, entitled “DUAL-AXIS ACCELEROMETER,” the entire disclosure of which is also hereby incorporated herein by reference.
Some conventional capacitive type accelerometers employ a vertical stacked structure to sense linear acceleration in the vertical direction. The stacked vertical structure typically has an inertial proof mass suspended between upper and lower fixed capacitive plates. The inertial proof mass moves upward or downward responsive to vertical acceleration. The measured change in capacitance between the proof mass and the fixed capacitive plates is indicative of the sensed acceleration. The vertical stacked structure employed in the aforementioned conventional linear accelerometer generally requires significant process complexities in the fabrication of the device using bulk and surface micro-machining techniques. As a consequence, conventional vertical sensing accelerometers typically suffer from high cost and undesired packaging sensitivity.
Recent efforts to advance the design and fabrication of accelerometers have included efforts to design a tri-axis acceleration microsensor. Prior known approaches have employed multi-sensing elements with special processing that typically requires both lateral and vertical oriented plates for capacitive sensing, and require a complex process to create sensing elements having complex angles that allow acceleration sensing in three different axes. Some proposed accelerometers have employed piezo-resistors included in the sensing device. Additionally, many prior known approaches employ electrode anchors fixed along the perimeter of the suspended proof mass, which results in unwanted sensitivity due to packaging induced stress.
Accordingly, it is therefore desirable to provide for an accelerometer that senses acceleration in multiple directions and does not suffer undesired packaging sensitivity and other drawbacks of prior known sensors. In particular, it is desirable to provide for a cost-effective linear accelerometer that may sense acceleration in multiple axes including both magnitude and direction of vertical acceleration.